1. Statement of the Technical Field
The invention concerns communications systems, and more particularly, the efficient digital generation of sine/cosine evaluations.
2. Description of the Related Art
Conventional sine/cosine generators typically compute a sine and cosine of an input angle. According to one communications system application, the sine and cosine of the same input angle can be used together to form a quadrature form of a sinusoid, which is the equivalent to the complex-valued exponential evaluated at the input angle. Quadrature forms of sinusoids are well known to those having ordinary skill in the art, and therefore will not be described herein. However, it should be understood that quadrature sinusoids are often used in digital up conversion applications, digital down conversion applications, numerically controlled oscillator applications, and Fourier transform applications.
One conventional method for generating a quadrature sinusoid employs look up tables (LUT). The LUT based method is well known to those having ordinary skill in the art, and therefore will not be described herein. However, it should be understood that the LUT based method generally involves mapping a fixed-precision input angle to a pre-defined evaluation of a trigonometric function within some resulting accuracy. The LUT based method generally provides reasonably efficient computation for ten to twelve (10-12) bit precision sine and cosine trigonometric computations. If precisions greater than ten to twelve (10-12) bits is desired, then the LUT based method requires exponentially increasing amounts of hardware for linear increases in bits of precision. Therefore, the LUT based method is hardware inefficient for high-accuracy applications.
Other conventional methods for generating a quadrature sinusoid employ Coordinate Rotation Digital Computer (CORDIC) algorithms. CORDIC algorithms generally use vector rotation to compute the sine and cosine of an input angle. CORDIC algorithms involve iteratively performing vector rotations by an arbitrary angle using shift and add techniques. As such, CORDIC based methods are computationally intensive. Further, the basic CORDIC method cannot take advantage of any natural symmetries in the underlying trigonometric operators. As such, the basic CORDIC method is less efficient than tailored hardware calculators.
Another conventional method of generating a quadrature sinusoid employs an out-of-phase algorithm. This out-of-phase based method is described in U.S. Pat. No. 5,276,633. This out-of-phase based method more efficiently computes the sine and cosine of an input angle as compared to the LUT and CORDIC based methods. In this regard, it should be understood that the out-of-phase based method provides ten to eighteen (10-18) bit precision sine and cosine trigonometric computations. However, such a ten to eighteen (10-18) bit precision is unsuitable for digital signal processing applications requiring a higher degree of accuracy.
In view of the forgoing, there is a need for an improved sine/cosine generator and method for efficient digital generation of a sine/cosine of an input angle. The improved method needs to be less computationally intensive than the conventional CORDIC based methods. The improved method also needs to provide a bit precision greater than the bit precisions of conventional LUT and out-of-phase based methods.